Sondes having neutron sources and neutron detectors are commonly used in well logging. The sonde is lowered on the end of a cable into a borehole of a formation and then is slowly winched toward the surface as measurements are made. the neutron source of the sonde generates neutrons through the components of the formation being logged. The neutrons collide with nuclei of the components of the formation in an elastic collision. The neutron detector of the sonde senses the neutrons that ricochet through the formation and back to the sonde. A record of the detected neutrons, a neutron log, is used in interpreting the porosity in combination with the density, lithology and gas of the formation.
Neutron detectors for well logging typically comprise a metal housing that contains .sup.3 He gas. Neutrons that reach the detector collide with atoms of the .sup.3 He gas. This collision usually produces a proton and a triton at the low neutron detection energies of neutron well-logging. At higher neutron energies, direct collisions with .sup.3 He nuclei are more likely. A biased conductor surrounded by the .sup.3 He gas within the housing is affected by the charge that results upon the release of the proton and the triton or upon the direct collisions. This produces an electrical pulse that travels through the conductor. The neutron detector counts each such pulse as an indication of the presence of a neutron. However, such electrical pulses have amplitudes that differ from one another and which determine a pulse height spectrum. The amplitude of each electrical pulse depends on the location and orientation of the associated particles when the collision occurs, for example.
Neutron detectors are also responsive to gamma radiation that occurs naturally in the formation being logged or is generated due to reactions of neutrons with the nuclei of the formation, borehole, or sonde. Gamma radiation can free electrons from the housing, which also produces ions in the .sup.3 He gas. The biased conductor within the housing is also affected by charges that result upon the release of the electrons and the ions. Each charge produces an electrical pulse that travels through the conductor. The neutron detector could falsely count each such pulse as an indication of the presence of a neutron. Because a neutron detector is responsive, not only to neutrons, but also to gamma radiation, such a neutron detector would indicate an inaccurate neutron count.
U.S. Pat. No. 4,476,391 to Bednarczyk for "Method for Improving Accuracy in a Neutron Detector" ("the Bednarczyk patent") concerns an approach for distinguishing between counts caused by neutrons and gamma radiation. This approach involves the setting of a predetermined amplitude threshold level. All counts that occur below this threshold level are removed from a total count by the neutron detector and are considered to be the result of gamma radiation. All counts that occur above this threshold level are included in the total count by the neutron detector and are considered to be the result of neutron collisions. However, this approach is open loop and is affected by many conditions. For example, temperature will affect the detector such that the threshold level and the bias of the conductor can drift independently. Thus, repeated neutron logs of the same formation could be inconsistent.
FIG. 1 is a schematic diagram of a prior art neutron detector 10 having a circuit that distinguishes between actual neutron counts and gamma radiation counts. The neutron detector 10 and circuit connect in an open loop. The neutron detector 10 contains .sup.3 He gas and is responsive to neutrons occurring in the formation. As described in the background of the invention, the neutron detector 10 is also responsive to electrons that have been freed by gamma radiation. A high voltage source 12 produces a set voltage level that biases a conductor (not shown) within the neutron detector 10. The neutrons and gamma rays produce pulses on the conductor, as described above. A resistor 14 blocks the pulses from reaching the high voltage source 12. A capacitor 16 blocks the voltage of the high voltage source 12 from reaching an amplifier 18.
The amplifier amplifies the input signal comprising the pulses from the conductor and produces an output signal, as described below concerning FIG. 2. A discriminator 20 sets a predetermined threshold level. Pulses having an amplitude above the threshold level are very likely produced by a neutron. Thus, pulses having an amplitude above the threshold level are counted as part of the neutron detector count rate. Pulses having an amplitude below the threshold level are very likely produced by noise or an electron that has been freed by a gamma ray, for instance. Thus, pulses having an amplitude below the threshold level are not counted as part of the neutron detector count rate.
FIG. 2 is a graph that illustrates a pulse height spectrum 22 of a .sup.3 He neutron detector, which is the output signal of the amplifier of FIG. 1. The number of pulses produced by the neutron detector 10 are plotted as counts on the abscissa. The amplitude of the pulses produced by the neutron detector 10 are plotted as channel numbers on the ordinate. The channel numbers are directly proportional to amplitude of the pulses. The large capture reaction peak of the signal represents those pulses caused by neutrons. The energy released is 0.76 MeV and is due to a capture reaction: EQU n+.sup.3 He.fwdarw.p+t
where n is the neutron, p is a proton, and t is a triton. This energy represents the binding energy of 2 protons and 1 neutron of .sup.3 He compared to the binding energy of 1 proton and 2 neutrons in a triton.
The second peak 24 is an artificial peak of the signal that represents noise and those pulses caused by electrons that have been freed by gamma radiation. The discriminator level is typically set to coincide with a minimum 26 in the pulse height spectrum, below which pulses caused by gamma radiation and noise occur, and above which pulses caused by neutrons occur.
The open loop system of FIG. 1 is affected by many conditions. For example, temperature will affect the detector 10 such that the threshold level and the bias of the conductor can drift, causing neutron logs of the formation to be very inaccurate. For example, if the voltage of the high voltage source 12 increases due to a reduction in load or a decrease in ambient temperature, the output of the amplifier 18 would change such that the pulse heights would appear to be larger for all of the counts made by the neutron detector 10. As a result, the spectrum of FIG. 2 would shift to the right. Given a stable discriminator level, the nuclear detector 10 would count a different portion of the pulse height spectrum 22.
Furthermore, the discriminator level could vary independently. For example, an increase in ambient temperature could cause the discriminator 20 to produce a higher threshold voltage level. As a result the discriminator level of FIG. 2 shifts to the right. The neutron detector 10 would count less of the FIG. 2 pulse height spectrum 22.
U.S. Pat. No. 3,922,541 to Seeman ("the Seeman patent") describes a method and apparatus for stabilizing the gain of a radiation detector. However, as described below, the method and apparatus of the Seeman patent are not directly applicable to the present invention.
According to the Seeman patent, a processing circuit outputs a spectrum of pulses to an amplifier that applies the pulses to three comparators, each having a reference amplitude. One amplitude corresponds to the summit of a reference peak. Two other amplitudes correspond to points at edges of the reference peak. The outputs of the three comparators connect through other circuitry to a high voltage power supply control unit. This unit sends a signal to adjust the high voltage of a voltage supply and thus the gain of the detector. The apparatus of the Seeman patent uses a gamma ray source, such as Cesium-137 to produce the reference peak. This reference peak, as the Seeman patent describes, is outside the spectrum of the pulses from the amplifier.
FIG. 3 is a graph illustrating the spectrum of pulses detected by a gamma counter, such as that of the Seeman patent. The abscissa plots energy level in electron volts and the ordinate plots the number of gamma ray counts. A first curve 28 indicates that most gamma ray counts occur in the 200 ke V range, and are negligible in the 500 ke V range. A second curve 30 indicates the counts produced by a source such as Cesium-137, which corresponds to the reference peak of the Seeman patent. These counts group closely at 660 ke V, well outside the spectrum of gamma ray counts 28 that occur in the formation. Because counts caused by the source are clustered in a stable grouping well outside the spectrum of gamma ray counts 28, the second curve 30 is chosen as a reference curve. Any shift in the spectrum is compared to the reference second curve 30.
The generation of a reference curve at an energy level significantly less than the 660 ke V level would coincide with and thus interfere with the counts represented by the first curve 28, and would provide an inaccurate count of gamma radiation. Therefore, the reference peak of the Seeman patent is deliberately set well outside the gamma pulse spectrum 28 to minimize interference with that spectrum.
The reference peak of the Seeman patent is deliberately set well outside the gamma pulse spectrum for another reason. Gamma radiation spectrums change shape because the amount of radiation that is sensed by the gamma detector is dependent on the environment of the detector. The composition of the formation surrounding the gamma detector drastically affects the levels of gamma radiation that pass through the formation to the detector and, thus, the gamma radiation spectrum. A third curve 32 is an example of another gamma radiation spectrum. It is the very difference between gamma radiation spectrums, such as 28, 32, that, when interpreted, indicates the composition of the formation. Because the gamma radiation spectrum 28, 32 depends on the composition of the formation and therefore changes shape, the reference peak 30 is set at 660 ke V, well outside any the range of any expected spectrum, so that the reference peak 30 does not coincide or interfere with the gamma radiation counts 28,32.
FIGS. 2A and 2B illustrate that a narrower high voltage operating plateau can result when high pressure detectors are used for epithermal detection, for example. FIG. 2A plots the normalized counts of the pulse height spectrum of a neutron detector. FIG. 2B plots normalized counts of a neutron detector as a function of voltage applied to that detector. A first curve 35A illustrates the counts of a low pressure neutron detector at 10 Atm, for example. The curve has a relatively long plateau 34A. Thus, a drift in voltage that occurs between voltage values C and D is within the plateau and the normalized output of the neutron detector changes negligibly. A second curve 35B illustrates the counts of a high pressure neutron detector at 40 Atm, for example. The curve has a relatively narrow plateau 34B. Thus, a drift in voltage that occurs between voltage values C and D is outside the plateau and the normalized output of the neutron detector can change drastically. Narrow high voltage operating plateaus also result when neutron detectors have short time constants for charge collection or when the geometry of the detector is less than a 3/1 ratio of length to diameter, for example.
High pressure detectors are also more sensitive to gamma radiation, which can cause the gamma radiation counts to increase to the line 36. The plateau 34B of FIG. 2B narrows as the gamma radiation counts encroach upon those made by neutrons, which are represented by the capture reaction peak.
Operating a neutron detection system under narrow plateau conditions has been unsatisfactory, because such a system is more sensitive to changes in the voltage level of the high voltage or the discriminator threshold voltage. A drift in the discriminator level under narrow plateau operating conditions would either incorrectly count gamma radiation as neutron counts if the discriminator level shifts to the left, or incorrectly exclude neutron counts if the discriminator level shifts to the right. Similarly, a drift in the high voltage level would either incorrectly exclude neutron counts if the high voltage level shifts the reference peak to the left, or would incorrectly count gamma radiation as neutron counts if the high voltage level shifts the reference peak to the right. Conversely, a system operating under longer plateau conditions would permit a larger margin of drift of the high voltage level and the discriminator level.
However, according to the present invention, the pulse height spectrum of a neutron detector is held constant relative to the discriminator level. Thus, operating a neutron detector under narrow plateau conditions is not critical to the accuracy of the detector.